Log warn repetition on 0.2.1

I changed the version from 0.2.0 to 0.2.1， then I got the log warn
```
log4j:WARN No appenders could be found for logger (com.alibaba.dubbo.common.logger.LoggerFactory).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
```
application.yml
```
dubbo:
application:
name: XX-service
id: XX-service
logger: slf4j
```